This invention generally relates to the reprocessing of spent nuclear reactor fuel and, more particularly, to techniques for preventing the unauthorized diversion of plutonium.
Fast breeder reactors are designed to extract useful energy from our natural resources, namely uranium and thorium. It is an essential part of the fuel cycle of these reactors to reprocess plutonium. Currently a number of countries including the United States, The United Kingdom, France, Germany, Russia, India and Japan are processing plutonium and purifying it by various processes. About a dozen additional countries have operated extraction processes on a laboratory or pilot scale. About thirty-two additional countries have nuclear research establishments with evident technical potential or capability for reprocessing plutonium.
There is serious concern throughout the world that the increasing deployment of reprocessing capacity for nuclear reactor fuel will increase the likelihood of further proliferation of nuclear weapons. This concern is due to the fact that essentially all processing methods used to date are derived from the processes originally developed during or shortly after World War II for producing plutonium for nuclear weapons. This concern has been led by the United States Government which since 1977 has taken the position that such processes are undesirable for civilian power use because they potentially make purified fissionable materials available and therefore susceptible to diversion by terrorist groups. In addition, there is concern because such reprocessing plants could readily be converted over to the extraction of weapons material by a change of intention by a government which had previously pledged by treaty to forego the production of nuclear weapons.
By way of background, FIG. 3 illustrates schematically one of the currently used processes for reprocessing spent nuclear fuel. The fuel is first removed from a nuclear reactor 5 and is stored for varying periods of time, typically at least three months, in order to allow the short-lived fission products to decay. This material 6 includes both spent fuel and, in the case of a breeder reactor, blanket material. The material is next transferred to a dissolver 7 where it is dissolved into a liquid solution 8. To facilitate dissolution of fuel, the cladding and fuel is cut up into small segments prior to transfer to the dissolver. This is called the "chop and leach" method of conversion.
The liquid solution 8, FIG. 3 from the dissolver 7 is then passed to a contacting device where the step of extraction 9 is performed. This step includes two or three extraction cycles at the end of which the uranium and plutonium are essentially completely separated from each other and from the accompanying radioactive fission products. A contactor is a mechanical device for bringing two liquid phases into contact and for causing the uranium and plutonium to be transferred from a water phase to an organic phase or conversely. The product stream 11 coming out of the extractor contains uranium and plutonium with only 0.01% to 0.1% of the fission products. The other discharge stream 12 from the extraction process is a waste stream containing nearly 100% of the fission products. The fission products in the waste stream are usually concentrated and stored, eventually to be solidified and stored 13 is a radioactive waste disposal area.
The product stream 11, FIG. 3 from the extraction process 9 goes to a second contactor where the step of partitioning 14 occurs. The contactors fractionate the uranium and plutonium into an essentially pure plutonium stream 15, and a pure uranium stream 16. The contactors make a plutonium stream 15 that contains less than 0.1% uranium and traces of fission products. The remaining traces of fission products and uranium are removed from the plutonium stream by second and third cycles 23. These cycles may be either further solvent extraction and stripping cycles, or alternate methods such as ion exchange absorption-elution cycles using resins or silica gel columna. Similarly, the uranium stream is subject to two or three added cycles of purification 25, usually using solvent extraction and stripping cycles, supplemented by fluoride volatility separation which can be conveniently obtained if the uranium is to be further processed in an isotope separation process using uranium hexafluoride. The pure plutonium stream 15 is then passed through subsequent processes 18 where a mixed oxide fuel 19 is fabricated. The mixed oxide fuel is then installed 20 back into the reactor for further power generation. The uranium stream may be used either for fuel fabrication, or converted to uranium hexafluoride 27 for feed to an isotope separation plant.
It should be appreciated that plutonium is produced at numerous points in this process which is sufficiently pure to be used for weapons material. The pure plutonium stream 15, FIG. 3 is one example. In addition, plutonium mixed with uranium and only low levels of fission products is present in the extraction product stream 11. Purified plutonium solutions or compounds offer the least obstacle to diversion because they can be transported with no more radiation shielding than that provided by ordinary containers. Detection by radiation detectors of theft or diversion is difficult and can be circumvented by small thicknesses of material. Pure plutonium emits only an alpha particle, a low-energy, low-abundance X-ray, and, from reactor-grade plutonium, a small flux of neutrons.
In the past coprocessing has been suggested to increase diversion resistance. In coprocessing the uranium and plutonium product streams are maintained together throughout the fuel recycling process by limiting the efficiency of the partitioning step 14 but with a high degree of separation from fission products. This increases the resistance to diversion somewhat, since further chemical steps are necessary to obtain weapons-usable plutonium. In addition, a greater bulk of material must be diverted in order to obtain sufficient material to make a weapon.
Of itself coprocessing does not provide sufficient improvement in diversion resistance. The product is still low in radioactivity and difficult to detect in ordinary containers if diversion is attempted. In addition, the usual process design and equipment layout provides for conveniently repeating process steps by returning a product stream to a feed tank for a second pass through part or all of the process. In FIG. 3 the paths for recycling streams are illustrated in dotted lines 17, 22, 23, 24 and 25. With such provisions the added diversion resistance effect of coprocessing can be thwarted by suitable recycling.
Another prior approach for increasing diversion resistance has been to perform the complete separation of uranium, plutonium, and fission products by a solvent extraction process as generally described in FIG. 3. Uranium and plutonium of high purity are produced in two separate streams and then mixed prior to storage, shipping, and fuel fabrication. The resultant stream, for example, 20% plutonium and 80% uranium by weight, cannot be directly used to fabricate a nuclear weapon.
This latter approach has limited benefit to diversion resistance since the detectibility of attempted diversion is not enhanced, and the chemical separation of plutonium from uranium is a relatively simple process in the absence of fission products. In addition, pure plutonium is potentially divertable within the plant before it is mixed into the uranium product stream.